Disables the gemfire plugin.

This patch comments out the gemfire plugin from the pom.xml as the
build fails with the following message:

[ERROR] Failed to execute goal on project gemfire-binding: Could not
resolve dependencies for project com.yahoo.ycsb:gemfire-binding:jar:0.1.4:
Failure to find com.gemstone.gemfire:gemfire:jar:6.6 in
http://dist.gemstone.com.s3.amazonaws.com/maven/release/ was cached in
the local repository, resolution will not be reattempted until the
update interval of gemstone has elapsed or updates are forced -> [Help 1]